The present invention relates to the field of large aperture ultrasonic transducers, and more particularly to transducers which are useful in ultrasonic imaging systems for imaging internal body tissues and for other related medical applications.
In the field of ultrasonic medical imaging, it has long been desired to achieve good lateral resolution, good penetration, good axial resolution, reasonable electrical impedance characteristics, and sufficient ruggedness to permit extended transducer use. Unfortunately, optimization of certain of the forementioned characteristics tends to prevent optimization of certain other characteristics which may be deemed necessary or desirable for a given application. For example, the degree of lateral resolution (d) which is possible using a transducer is defined by the Rayleigh criterion as being inversely proportional to aperture size (a), and directly proportional to the focal length of the optical system and the wave length of the radiation involved. Accordingly, in the field of medical imaging, where good lateral resolution is preferred, and where trapezoidal images are to be avoided during sector scanning, relatively large apertures (e.g., greater than 1.5 inches (3.8 cm) and, on the order of about 3 inches (7.6 cm)) are desired. At the same time, it is often difficult to achieve the short impulse response which is required to achieve good axial resolution. In the absence of short impulse responses, reflected pulses from neighboring but distinct tissues tend to overlap due to the long train of oscillations (long ring down time) associated with long impulse responses. Since resolution is directly proportional to wave length, to some degree resolution can be increased at higher transducer frequencies, however, higher frequencies are not as effective at penetrating body tissue, tend to cause shadowing effects within those tissues and require much higher sensitivities in order to effectively image deep lying tissues without having to use high intensity ultrasound. In optimizing each of the above-mentioned parameters, it is further necessary to insure that the resulting transducer exhibits a reasonable electrical impedance which enables the transducer to be matched to a driving source without undue effort.
In recent years, various large aperture transducers suitable for medical applications have been suggested, several of which have achieved a certain degree of commercial success. In "Transducer Arrays Suitable For Acoustic Imaging", by C. S. DeSillets, G.L. Report No. 2833, Stanford University, California (1978) a relatively small apertured transducer is disclosed having a lead metaniobate active element. DeSillets' transducer is air backed, is designed for operation at 2.06 MHz, exhibits a real impedance at f.sub.o of 64 ohms, and exhibits a round trip loss of 6.5 dB. Using a 1/4 wave length matching layer of DER 332 epoxy, this transducer exhibits an estimated ring down time to 40 dB of about 3.25 microseconds.
More recently, large aperture transducers have been suggested which exhibit comparable or better ring down times than the aforementioned DeSillets' transducer. Such transducers have typically featured piezoelectric materials exhibiting good electromechanical couplings (high k.sup.2.sbsp.T values) and suitable dielectric contants for their intended applications. Presently available piezoelectric materials have coupling coefficients (k.sup.2.sbsp.T) in the range of 0.02 for PVDF to 0.26 (for PZT-4). Lead metaniobate active elements similar to that disclosed by DeSillets exhibit k.sup.2.sbsp.T values in the 0.122 to 0.144 range. These materials, however, have either too low a coupling coefficient or too high a dielectric constant and are thus unsuitable for large aperture transducers.
Lithium niobate is the presently preferred material for use in large cross-section transducers. Lithium niobate has been typically used as the active element in transducers used for SAW (surface acoustic wave) applications, and more recently has been used as the active element in ultrasound imaging transducers. In one such transducer, a lithium niobate active element having a thickness of about 1.05 mm, and designed to operate at 3.2 MHz was utilized in a transducer having an impregnated epoxy backing and two matching layers having impedances of 10.6 and 3.1 kg/m.sup.2 sec. Such transducers were typically able to achieve ring down times to 40 dB in the range of 1.4 to 1.7 microseconds.
One method which has been employed by the art in designing ultrasonic transducers is the use of computer modeling programs which are based on acoustic theory and which attempt to predict the theoretical performance of tranducers which are constructed from various active elements and various matching materials. Such approaches at optimizing transducer performance have been described in papers by G. Kino, et al such as the paper entitled "The Design of Broadband and Efficient Acoustic Wave Transducers", presented at the IEEE Conference on Sonics and Ultrasonics (1980). Such programs generally take into account transducer impedance, matching layer thickness, backing properties, etc. for the purpose of optimizing transducer performance. The application of such programs has caused the substantial improvement in such parameters as ring down. For example, in one experiment conducted at Stanford University (as reported in the above paper) wherein a transducer was constructed in accordance with computer modeling theories, a 3.5 MHz transducer, 2 mm in diameter, was improved in ring down from 15 to 5 cycles.
Notwithstanding the theoretical advances in this area, the prior art has yet to achieve a transducer exhibiting a round trip 40 dB ring down time of less than 1 microsecond at 4.2 MHz. In this regard, applicant has recently tested an ECHO transducer E81X414 (3.5 MHz) to determine its round trip loss and 40 dB ring down time. This transducer was tested by generating a tone burst in the transducer of about 10 cycles in duration having an amplitude of 10-15 volts. This tone burst was transmitted through water reflecting it off a stainless steel plate and measuring the amplitude of the return pulse as well as the time it takes for the return pulse to decay down 40 dB from the last maximum. This test is conducted with the subject transducer located 10 cm away from a stainless steel plate placed in the focal plane of the transducers. The effect of the stainless steel plate was subtracted from the round trip loss (0.6 dB), so that a direct comparison can be made with the transducers disclosed hereinafter. Under the subject conditions, the ECHO transducer was found to exhibit a round trip loss of 10.0 dB, a 40 dB ring down time of 1.3 microseconds or 4.5 cycles in the ring down time. For these tests, a series inductor/transformer electrical matching network was used. Based on this evaluation, the ECHO transducer is believed to represent the state of the art prior to the inventions disclosed herein, and therefore to be comparable to the best commercially available medical imaging transducers. Other transducers, such as Toray transducer #SN-35M-850, which are believed to utilize PVF.sub.2 active elements have also been found to exhibit a 40 dB ring down time of 1.3 microseconds, even though the round trip loss for such transducers has been measured to be in the range of about 19.4 dB. This transducer also exhibited an envelope of oscillations present about 1 microsecond beyond the point of 40 dB decay, which extended for 7 microseconds and was about 34 dB down from the last maximum. Such transducers are thus less preferred for use in medical imaging applications.